Thursday, June 27, 2013

Walk this way

I recently found a handy mechanism for walking a directory tree in Linux. In
general, the way I used to do this was to use facilities found in dirent.h and
write my own recursive directory walker. Something similar to:

#include <stdio.h>
#include <string.h>
#include <dirent.h>

void reclist (const char* dirname) {

    DIR* dir = opendir (dirname);
    struct dirent* entry = 0;
    char name[1024] = {0};

    if (! dir) { return; }

    entry = readdir (dir);
    while (entry) {
        if (strncmp (entry->d_name, ".", 1)) {
            switch (entry->d_type) {
                case DT_REG:
                    printf ("%s\n", entry->d_name);
                case DT_DIR:
                    snprintf (name, 1024, "%s/%s", dirname, entry->d_name);
                    reclist (name);
        entry = readdir (dir);
    closedir (dir);

int main(int argc, char** argv) {
    const char * dir = ".";
    if (argc == 2) { dir = argv[1]; }
    reclist (dir);
    return 0;

While that does work, it is rather verbose (especially once you get used to
environments like Ruby and Python). It turns out that ftw.h provides a more
concise way to do the above while managing all the little details like
avoiding '.' and '..' and managing the current path string. Here is what that
looks like to do the same as the above:

#include <stdio.h>
#include <ftw.h>

int handle_entry (const char *entry, const struct stat *sb, int type) {
    if (type == FTW_F) {
        printf("%s\n", entry);
    return 0;

int main() {
    ftw(".", handle_entry, 10);
    return 0;

I also like the fact that a callback is used to operate on each of the files
found. It makes managing changes much easier as the tree walking is separated
from the code that handles the logic associated with inspecting the files.

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